The serotonin (5-HT) transporter (SERT) is responsible for terminating serotonergic neurotransmission by high-affinity uptake of 5-HT from extracellular fluid (ECF) and is therefore critical in determining concentrations of 5-HT in ECF. Drugs that act at the SERT, such as selective serotonin reuptake inhibitors, ameliorate several neuropsychiatric disorders, including depression and certain anxiety disorders. The SERT is also a primary site of action of drugs of abuse such as "Ecstasy" (MDMA). However, there is remarkable variability in the response of individuals to these drugs and the reason for this is not clear. For example, some individuals respond well to antidepressant treatment while others do not. Also, certain people will overdose on MDMA at a dose that will only mildly affect others. Allelic variations in the SERT have now been identified that influence SERT activity and may be associated with a number of psychiatric disorders and/or to an individual's response to drug treatment. The main goal of this proposal is to study the functional and adaptive consequences of genetically- induced reductions in the expression of the SERT and 5-HT1B receptor on the clearance of 5-HT from ECF in vivo. Using in vivo high-speed chronoamperometry we have found that the 5-HT1B autoreceptor can regulate clearance of 5-HT from ECF and also, that alterations in the density of the SERT can change the clearance rate of 5-HT. This proposal will take advantage of two lines of mice with null mutations of the SERT or the 5-HT1B receptor gene (the SERT knockout (KO) and 5-HT1B receptor KO mice), to gain new insight into both the regulation of the SERT and the effect of altered gene expression on drug sensitivity in these animals in vivo. It is predicted that in heterozygotes of both KO strains (which express 50 percent fewer SERTs and 5-HT1B receptors respectively), clearance rates of 5-HT will not differ from wild-type mice under basal conditions, but defects will be revealed in response to pharmacologic challenge. Our general hypothesis is that changes in the kinetics of 5-HT clearance as an adaptive consequence of reduced SERT or 5-HT1B receptor density will alter the sensitivity of these mice to psychotropic drugs. Changes in sensitivity will be indexed by the ability of these drugs to influence the kinetics (KT and Vmax) of 5-HT clearance in the three genotypes of each KO strain. High-speed chronoamperometry will be used to measure 5-HT clearance in vivo. Quantitative autoradiography will be used to assess the effect of genetic mutation of the SERT on 5-HT1B receptor density and vice- versa. Alterations in the uptake of 5-HT by these mice and in the response of these mice to psychotropic drugs will provide information important to our better understanding of the pathobiology of neuropsychiatric disorders and addiction as well as to the selection of drug treatment in individuals with allelic variations of the SERT.